Arm resets position when exported?
The mesh uses a modifier to deform based on the position of the armature. By default the deformation from the armature is not visible while in edit mode but can be turned on if you want to. The button highlighted in red will show a 'shadowy' version of where the mesh would be based on the armature. The blue highlighted button is only visible if the first one is enabled and when enable the mesh will be shown fully deformed while in edit mode.



As for Unity - check that you are exporting the action and that Unity is using the action and not just the default pose position.